1. Field of the Invention
This invention relates to plasma for processing substrates and more particularly, to high density plasmas for etching substrates and for plasma enhanced chemical vapor deposition (CVD) on substrates.
2. Description of the Prior Art
A low temperature plasma (or process plasma) excited by radio frequency or microwave power has been widely used to fabricate microelectronic devices. A recent trend in silicon technology demands a single wafer process for both etching and thin film deposition in view of full automation of an ultra large scale integrated circuit fabrication line. In a single wafer process, the wafer throughput must be high in order to be economical. This means that a high radio frequency or microwave power source is needed to achieve a fast etch rate or deposition rate. High power operation, however, accompanies negative adverse effects such as radiation damage and particulate contamination to the substrate which becomes more and more of a critical issue as device dimensions continue to shrink in silicon technology.
In order to cope with the adverse effects of radiation damage and particulate contamination, Electron Cyclotron Resonance (ECR) of the microwave plasmas under a steady magnetic field have been applied for both etching and deposition. However, the ECR plasma density is not sufficient enough to take full advantage in utilizing a low pressure plasma below 1.0 mTorr. The normal operating pressure of plasmas generated by ECR is around a 10 mTorr range in order to achieve a reasonable wafer or substrate throughput.
In U.S. Pat. No. 4,683,838 which issued on Aug. 4, 1987, to Kimura et al., a plasma treatment system is disclosed for forming insulator films by making a stream of evaporated metal atoms generated from an evaporation source cross a stream of plasma containing highly activated oxygen atoms or nitrogen atoms at a high concentration. The plasma is formed in a discharge tube filled with a dilute gas by a microwave power source generated by a magnetron at 2.45 GHz which is led to the discharge tube. The discharge tube is surrounded by electromagnets whereby the charged particles in the plasma are put in a spiral motion so as to coil around the magnetic flux. The frequency of spiral motion is proportional to the mass of charged particles and the intensity of the external magnetic field. When the frequency of spiral motion becomes equal to that of the microwave, the charged particles will absorb the microwave to increase the kinetic energy. To make the electrons absorb the microwave, it is necessary to use an external magnetic field of 875 gauss. The electric discharge can be continuously and stably carried out even in a dilute gas atmosphere of below 10.sup.5 Torr, and dissociation, excitation, ionization, etc of gas molecules can efficiently take place owing to a high kinetic energy level of electrons as compared with DC or RF discharge of 13.56 MHz.
In U.S. Pat. No. 4,691,662, which issued on Sep. 8, 1987, to Roppel et al., a dual plasma microwave apparatus is described wherein a first disk plasma is generated. A grid or screen having an electrical bias removes ions or electrons from the first disk plasma to a second location forming a hybrid plasma. The hybrid plasma is used to treat a surface of an article in a different manner than the first disk plasma.
In U.S. Pat. No. 4,492,620, which issued on Jan. 8, 1985, to Matsuo et al., a plasma deposition method and apparatus is described for depositing a thin film of various materials over the surface of a substrate. A plasma deposition apparatus is described comprising a plasma formation chamber into which a gas is introduced to produce a plasma; a specimen chamber in which a specimen substrate table is disposed for placing thereon a specimen substrate on which a thin film is to be formed and for depositing the thin film on the specimen substrate; a plasma extraction window interposed between the plasma formation chamber and the specimen chamber; the target which is made of a sputtering material and is interposed between the plasma extraction window and the specimen substrate table; a first means for extracting ions for sputtering the target from a plasma stream produced in the plasma formation chamber; and a second means for extracting the plasma stream from the plasma extraction window into the specimen chamber and for transporting the sputtered and ionized atoms to the specimen substrate placed on the specimen table.
In U.S. Pat. No. 4,609,428, which issued on Sep. 2, 1986, to Fujimura, a microwave plasma etching method and apparatus is described having an improved anisotropic etching capability and processing rate for etching and ashing integrated circuit semiconductor substrates. The gas pressure in the apparatus as described is selected to be low, approximately 10.sup.-3 to 10.sup.-4 Torr, so that the mean free path of the gas molecules substantially exceeds the dimensions of the apparatus.
In U.S. Pat. No. 4,559,100, which issued on Dec. 17, 1985, to Ninomiya et al., a microwave plasma etching apparatus is described which is suitable for rapid etching of Si, SiO.sub.2, W, Al, etc. The microwave plasma etching apparatus comprises a discharge tube into which a discharge gas is supplied and which forms a discharge region; means for generating a magnetic field in the discharge region; means for bringing a microwave into the discharge region; and a stage for holding a material.
In U.S. Pat. No. 4,543,465, which issued on Sep. 24, 1985, to Sakudo et al., a microwave plasma source is described comprising a discharge space supplied with a microwave electric field and a DC magnetic field. A switch is provided for effecting through switching operation the change over a magnetic field applied to the discharge space from the intensity for the ignition of plasma to the intensity for ion extraction in succession to completion of the plasma ignition.
In U.S. Pat. No. 4,401,054, which issued on Aug. 30, 1983, to Matsuo et al., a plasma deposition apparatus as described having a plasma formation chamber and a specimen chamber which are arranged separately. Gaseous material and microwave power are introduced to the plasma formation chamber to generate plasma by a microwave discharge through electron cyclotron resonance. The plasma is extracted to the specimen chamber from the plasma extracting orifice.
In a publication by K. Suzuki et al., entitled "Radio-Frequency Biased Microwave Plasma Etching Technique: A Method To Increase SiO.sub.2 Etch Rate", J. Vac. Sci. Technol. B3, 1025 (1985), a microwave plasma etching apparatus is shown with an RF voltage supply having an output coupled to the substrate. The RF voltage permits ions to be accelerated to the substrate even if insulators exist on the substrate surface. Further, ions can be accelerated to the substrate surface without retaining their charge, since ions and electrons are accelerated to the surface in turn.
In a publication by R. W. Boswell et al., entitled "Pulsed High Rate Plasma Etching With Variable Si/SiO.sub.2 Selectivity And Variable Si Etch Profiles", Appl. Phys. Lett. 47, 1095 (1985), very high etch rates of Si in SF.sub.6 have been obtained in a low-pressure resonant RF discharge. The vacuum pump maintains the pressure in the interaction region of 7 mTorr.